In the current art a number of systems and methodologies exist for the localization of ground vehicles and aircraft (e.g., targets) in and around air traffic surveillance systems which can include airports, terminal areas and en route systems.
Many large airports utilize approach radar systems to locate and track targets outside the airport. These radar systems obtain good azimuth accuracy by using the narrow beam available from the large aperture antenna, whereas the range is calculated from the round trip delay of the signal from the radar to the target and back. Typically these approach radar systems require large rotating antennas making them expensive. In addition, these radar systems characteristically have an update rate of approximately 4.5 seconds, and consequently the response speed of the associated analysis equipment is limited by the update rate of the radar.
A second method for target localization is multilateration. Multilateration systems typically are made up of an arrangement of beacon transmitters and receivers. Multilateration is a Time Differential of Arrival (TDOA) technique that uses information from aircraft transponder transmissions to determine the precise location of a target. The algorithm for multilateration typically starts by estimating, using TDOA information, the approximate location of a target in either a two-dimensional or three-dimensional coordinate system. An optimization process is then performed around the approximate location of the target to provide a more accurate location of the target.
Multilateration systems can be used to locate and track targets on the ground at the airport for runway incursion and can also be used for locating and tracking approaching aircraft within relative close proximity to an airport and by en route air traffic surveillance systems. However, if it is required to provide large area coverage around the multilateration system, the ratio of the antenna baseline (distance between the receivers) to the range of the target becomes such that the Geometric Dilution of Precision (GDOP) becomes quite large. It therefore becomes more difficult to locate the target within the “ellipse of uncertainty” and the effectiveness of the multilateration system is significantly degraded as the distance from the system to the target increases.
One method to overcome this problem with the current multilateration technique is to install external antenna elements outside the boundaries of the multilateration system to increase the size of the antenna baseline. This, however, creates difficulties with regard to purchasing additional real estate, providing security outside of the system, maintenance of the external antenna elements, communication between the external antenna elements and the system, and other logistical issues.
While many air traffic surveillance systems are using one, or a combination of the above-detailed methodologies, some airports currently have no local, reliable methodologies for locating approaching aircraft, or vehicles on the ground for runway incursion. Some of these airports have adopted the practice of acquiring radar data from larger, better-equipped airports in their vicinity to provide information on the air traffic in their area. While this process can be beneficial to airports that would otherwise not have this information available, it is possible for targets of interest to the dependent airport to be obscured from the providing airport's radar by line of sight obstructions.
A need exists, therefore, for a reliable, relatively low cost solution to provide an air traffic surveillance system with the capability to locate targets en route, locate targets approaching the airport, locate targets on the ground as a component of a runway incursion system, improve the handover between approach systems and runway incursion systems, and to extend the range and accuracy of target localization utilizing existing air traffic surveillance systems without the need to install antenna elements outside the system boundaries.